<p>TF 4 is plain old hypo (sodium thiosulfate) and keeps till exhausted. I dilute it 1:4 or 1 lb. per gallon of water if using anhydrous crystals, It's slower acting than TF5 (ammonium thiosulfate) and I Fix for about 10 minutes or till the film clears. I pour it back in the storage bottle and reuse it until after it starts becoming milky then dump it. It's safe to use and also used in swimming pool care. It's also quite inexpensive and easy to mix but has smell which some people object to which I personal don't mind at all. I usually wash for at 10 to 20 minutes then soak the film In a wetting agent for 30 seconds and hang to dry. clothes pins work fine top and bottom. Some, like me, will wipe the excess water off. This is how I've done it for at least 60 years. My old negatives are still fine without stains.<br>
Bill</p>